docs: def remove_start_NaN(self, data, var=None):
          """ Remove start NaN.
          
          CHECK: Note issue with multi-column df.
          
          Parameters
          ----------
          data    : pd.DataFrame()
              Input dataframe.
          var     : list(str)
              List that specifies specific columns of dataframe.
  
          Returns
          -------
          pd.DataFrame()
              Dataframe starting from its first valid index.
         
          """
  
          # Limit to one or some variables
          if var:
              start_ok_data = data[var].first_valid_index()
          else:
              start_ok_data = data.first_valid_index()
        
          data = data.loc[start_ok_data:, :]
          return data